Process for heat treating cast maraging steels



United States Patent 3,341,372 PROCESS FOR HEAT TREATING CAST MARAGING STEELS Edward P. Sadowski, Ringwood, N.J., assignor to The International Nickel Company, Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed July 12, 1965, Ser. No. 471,458 4 Claims. (Cl. 148142) The present invention relates to a process for heat treating ferrous-base alloys and more particularly to a process wherein cast steels of the maraging type are subjected to a sequence of heat treating operations whereby the capability of such steels to resist stress-corrosion cracking is markedly improved.

As is known to those skilled in the art, the introduction a few years ago of the type of ferrous-base alloys popularly known as maraging steels, described generally in US. Patents Nos. 3,093,518 and 3,093,519, spawned a new dimension in ferrous metallurgy. The outstanding mechanical properties characteristic of these steels, particularly the combination of strength and toughness thereof, coupled with various processing advantages led to their rather spontaneous commercial acceptance. In view of such attributes, the pace of research has been accelerated with the objectives of further improving the properties of such steels and also to provide steels for specific applications. Notable in this regard, cast maraging steels of special composition have been developed asset forth in US. Patent No. 3,132,937 and these cast steels, when homogenized at 2100 F. and thereafter aged at 900 F. for about three hours, have been known to afford yield strengths on the order of about 240,000 pounds per square inch (p.s.i.) together with tensile elongations of about 10% in section sizes up to about one inch. As will be appreciated, such a combination of properties for a cast steel is quite attractive.

In the commercial exploitation of the above-discussed cast steels, efforts have been intensified to achieve a comparable magnitude of mechanical properties in thicker sections, e.g., three inches, and, of considerable importance, to obtain a cast maraging steel manifesting greater resistance to stress-corrosion cracking, particularly in marine environments. While the former has been attained via recent technological developments, stress-corrosion cracking characteristics have still been found Wanting and this is one of the specific objectives to which the present invention is directed. It should also be mentioned that the recommended homogenizing treatment of .2100 F. in US. Patent No. 3,132,937 operated as a competitive deterrent to many foundries not equipped with the necessary furnaces to operate at this high temperature.

It has now been discovered that the resistance to stresscorrosion cracking of cast maraging steels described herein can be greatly enhanced through the application of a special sequence of heat treating operations. This marked improvement obtains without significant impairment of mechanical properties.

It is an object of the present invention to provide a special heat treatment cycle to improve the stress-corrosion cracking of maraging steels in the cast condition.

Other objects and advantages will become apparent from the following description.

Generally speaking, the present invention contemplates subjecting a cast maraging steel containing about 14% to about 17.5% nickel, about 8% to about 12% cobalt, about 4% to about molybdenum, up to about 0.45% or 0.5% titanium, up to about 0.5% aluminum, e.g., about 0.03% to about 0.45% aluminum, up to about 0.05% carbon, up to 0.1% zirconium and the balance being essentially iron to a heat treatment cycle which includes (a) heating the steel at a temperature of about 1700 F. to about 1850 F. for a period sufficient to achieve good homogenization of the cast structure, e.g., about 1 to 10 hours, (b) cooling the'steel, for example, by air cooling, to a temperature sufiiciently low to provide a martensitic structure, e.g., below about 250 F. or 300 F., (c) subjecting the steel to a second stage heating (also referred to herein as a conditioning treatment) at a temperature of about 1000 F. to about 1250 F. for about one to eight hours followed by (d) a third stage heat treatment (also referred to herein as a solution annealing treatment) at a temperature of about 1350 F. to about 1600 F. while holding thereat for one to ten hours, e.g., about one hour per inch of thickness, (e) again cooling to achieve a martensitic structure, (f) thereafter aging the alloy at a temperature of about 800 F. to about 1000 F. for up to about 24 hours, e.g., one to ten hours and (g) finally cooling while retaining the martensitic structure formed as a result of the cooling step employed subsequent to the solution annealing treatment.

In addition to the constituents above enumerated, the steels can also contain up to about 3.5% chromium, which can be used to replace an equivalent percentage of nickel on a weight to weight basis. However, the sum of the nickel plus chromium should not exceed 17.5% and the nickel content should not be less than about 14%. Small amounts of supplemental or auxiliary elements can be present including up to about 2% tungsten, up to about 0.5 columbium, up to about 0.45% vanadium, up to about 0.5 tantalum, up to about 3% copper and up to about 0.3% beryllium. The supplemental elements which can contribute to the hardness or strength of the steels should not exceed a total of about 4% and preferably should not exceed a total of 3%. Deoxidizing and/or malleabilizing elements can also be present in small amounts, including up to about 0.05 boron, e.g., up to 0.03% boron, up to 0.2% of each of silicon and manganese, rare earth elements and other such constituents such as lithium, magnesium and uranium. However, impurities such as sulfur, phosphorus, nitrogen, oxygen, antimony, tin, selenium, tellurium, arsenic and bismuth should be kept at low levels consistent with good commercial steelmaking practice. In this regard, the steels can be prepared in the manner described in US. Patent No. 3,132,937.

A particular satisfactory alloy composition is as follows: 16% to 17.5% nickel, 9.5% to 11.5% cobalt, 4.4% to 5% molybdenum, 0.1% to 0.45 titanium, 0.03% to 0.45% aluminum, up to about 0.03% carbon, up to about 0.1% zirconium, with the balance being essentially iron. The compositions herein described are those contemplated in US. Patent No. 3,132,937.

In carrying the present invention into practice, the tem perature of the homogenization treatment should not fall below that necessary to insure the occurrence of recrystallization and a temperature of at least 1750 F. is preferred. While the complete theory is not yet at hand which might explain the phenomenon responsible for the striking improvement in stress-corrosion cracking behavior obtainable in accordance herewith, it is considered that the unusually small grain size, about ASTM No. 6 or finsteel development described in US. Patent No. 3,132,937, this is quite unexpected. In that patent, the recommended optimum homogenizing temperature was 2100 F., it being preferred not to depart more than 50 F. therefrom. However, such a temperature, insofar as the present invention is concerned, results in a grain size which is, comparatively speaking, quite coarse, em on the order of grain size ASTM No. 1 or coarser. In any event, stresscorrosion characteristics are seriously and adversely af fected by the utilization of such a homogenizing temperature. Indeed, the maximum homogenizing temperature should not exceed 1850 F. and a temperature range for the first stage of the heat treatment cycle is preferably from 1750 F. to 1825 F.

The decrease in homogenization temperature as discussed above is also most advantageous for other commercial reasons. As mentioned herein, many commercial producers, e.g., foundries, do not have the necessary furnaces or furnace equipment for carrying out the homogenization treatment at the high temperature of the order of 2100 F. Thus, they were faced with the problem of either investing additional capital to obtain such equipment or leaving the commercial market to others. This dilemma has been obviated.

The cooling operation immediately subsequent to the homogenizing treatment must be carried down to a temperature at which a good martensitic structure is obtained; otherwise, inferior properties result. In this regard,

the austenite to go back into solution such that it transforms to martensite on cooling before aging. While a solution anneal over the range of 1350 F. to 1600 F. is satisfactory, a temperature range of about 1400 F. to 1550" F. with a holding time of about one to eight hours is preferred. Following the solution anneal, the steels are cooled until a martensitic structure is obtained and are thereafter aged.

With regard to the aging treatment, a temperature of from 800 F. to about 1000 F. can be employed although it is preferred to use a temperature within a range of about 850 F. to 950 F. In this connection, it might be mentioned that temperatures appreciably below 800 F. require an inordinately long time to achieve a sufficient aging response and even then a full aging might not be achieved. On the other hand, temperatures of above about 1000 F. tend to promote austenite reversion which can result in lower strengths and hardness.

For the purpose of giving those skilled in the art a better understanding of the invention, the following illustrative data is given regarding the alloy compositions it is quite in order, and indeed preferable, to cool the given in Table I:

TABLE I Percent Alloy No.

N1 Co Me Al T1 C Si Mn Fe 1 16.4 10.5 4.7 0.17 0.44 0.005 0.01 0.05 Bal. 2 10.0 10.6 4.6 0.08 0.35 0.004 0.03 0.05 Bal.

1 Balance is iron plus impurities.

steels to room temperature before initiating the second stage or conditioning heat treatment.

The conditioning treatment is most essential, for without it, a fine grain structure is not obtained and stresscorrosion characteristics are poor. It would appear that this treatment results in a condition whereby retained austenite is present or the martensite formed as a result of the first cooling step reverts to austenite. Proceeding further, it is considered that this austenite forms additional nuclei for the ultimate obtaining of a fine grain size, the austenite co-existing with other phases. The minimum temperature for the second stage heating should not fall below a temperature of about 1000 F.; otherwise, insufficient nuclei would be present for the formation of new grains and a temperature of at least 1050 F. or 1075 F. is most advantageous since such temperatures promote finer grains, e.g., ASTM No. 8. On the other hand, temperatures above 1250 F. can lead to inferior results since such temperatures seemingly tend to contribute to coarser grains and thus such temperatures should be avoided. It is advantageous that the second stage heat treating operation be conducted over a temperature of about 1050 F. to 1150 F. for about one hour to six hours.

, As in the case of the first cooling operation, the steels can be cooled to room temperature before subjecting them to the third heating operation of the four-stage cycle. However, if desired, the steels can be immediately brought to the temperature range over which the solution annealing operation is conducted.

In respect of the solution annealing or third stage heat treatment, the yield and ultimate tensile strengths are seriously impaired should this step be omitted and this is deemed attributable to an austenite effect. The retained or reverted austenite brought about by the conditioning treatment is characterized by stability. In other words, if the steels are cooled from the conditioning treatment and then aged (or are directly aged), excessive amounts of austenite are still present because being stable the austenite does not transform to martensite. Now this austenite is much softer and weaker than, say, martensite and, as a consequence, lower strengths ensue. However, it is considered that the third stage heat treatment causes Each of the alloys inch in thickness) was subjected to a heat treatment in accordance with the invention (Heat Treatment A) and also in accordance with the recommended heat treatment set forth in US. Patent No. 3,132,937, the latter consisting of homogenizing at 2100 F. for four hours, air cooling to room temperature and then aging at 900 F. for three hours followed by air cooling (Heat Treatment B). Heat Treatment A consisted of homogenizing at 1800 F. for four hours, air cooling to room temperature, heating to a temperature of 1100 F. and holding for about four hours, transferring to a furnace at 1500 F. and holding thereat for about one hour, air cooling to room temperature, aging at 900 F. for about three hours and finally air cooling to room temperature.

Each of the steels was subjected to what is known to be a severe stress-corrosion cracking environment. This consisted of immersing the specimens in a solution of 3.5% sodium chloride in distilled water, the solution being aerated at room temperature. The standard three point load test was employed with each of the specimens being stressed to 90% of their yield strength (0.2% offset). Three specimens (and therefor three tests) of each alloy composition were tested and the results are reported in Table II:

b Average number of days in test before stress-corrosion earcking O serve.

2 Test discontinued after 90 days, no failure occurred in any of the three specimens.

The foregoing data are illustrative of the marked superiority regarding the resistance to stress-corrosion cracking of steels treated in accordance with the invenj tion and the same steels treated in accordance with the treatment recommended in US. Patent No. 3,132,937.

TABLE III a Alloy Heat Y.S., 0.2% U.T.S., Elon- Reduction N 0. Treatofiset, p.s.i. gation, of Area, ment p.s.i. percent percent 1 Modified treatment.

Table III illustrates the significant loss in toughness characteristics as reflected by the substantial drop in tensile elongation and severe loss in reduction of area resulting from the application of modified Heat Treatment B. Steels Nos. 1 and 2 having a section size of inch when subjected to Heat Treatment A in accordance with the invention manifested the same order of magnitude in mechanical properties as those obtained in connection with Alloy No. 3 having a thickness of three inches. Thus, no appreciable sacrifice in mechanical characteristics is experienced in achieving the high resistance to stress-corrosion cracking in accordance herewith.

A distinct advantage of the present invention is that cast steels of the maraging type are greatly less susceptible to stress-corrosion cracking in marine environments, particularly sea water and ambient atmospheres. In addition, the cast steels can be used in such applications as gun parts, tools, rolls, dies, etc.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

I claim:

1. A process for improving the resistance to stresscorrosion cracking of martensitic cast steels containing about 14% to about 17.5% nickel, about 8% to about 12% cobalt, about 4% to about 5% molybdenum, up to about 0.5% titanium, up to about 0.5% aluminum, up to about 0.05% carbon, up to 0.1% zirconium, up to about 3.5% chromium with the sum of nickel plus chromium not exceeding 17.5%, up to about 2% tungsten,

up to about 0.5% columbium, up to about 0.45% vanadium, up to about 0.5% tantalum, up to about 3% copper, up to about 0.3% beryllium, the total sum of the tungsten, columbiuin, vanadium, tantalum, copper and beryllium not exceeding about 4%, and the balance being essentially iron which comprises subjecting the steels to a homogenization heat treatment Within the temperature range of 1700 F. to 1850 F. for a period sufiicient to achieve a good homogeneous structure, cooling the said steel to a temperature sufiiciently loW to provide a martensitic structure, heating the steel within a temperature range of about 1000 F. to 1250 F. for about one to eight hours, solution annealing the steel at a temperature of 1350 F. to 1600 F. for about one to ten hours, again cooling to a temperature sufliciently low to provide a martensitic structure and thereafter aging the steels at a temperature of about 800 F. to about 1000 F. up to about 24 hours.

2. The process as set forth in claim 1 in which the cast steel subjected to heat treatment consists essentially of about 16% to 17.5% nickel, about 9.5% to 11.5% cobalt, about 4.4% to 5% molybdenum, about 0.1% to 0.45% titanium, about 0.03% to 0.45% aluminum, up to about 0.03% carbon, up to about 0.1% zirconium, the balance essentially iron.

3. A process for improving the resistance to stresscorrosion cracking of martensitic cast steels containing about 14% to about 17.5% nickel, about 8% to about 12% cobalt, about 4% to about 5% molybdenum, up to about 0.5% titanium, up to about 0.5% aluminum, up to about 0.05% carbon, the balance essentially iron which comprises subjecting the steels to a homogenization heat treatment within the temperature range of1750" F. to 1825 F. for a period sufiicient to achieve a good homogencous structure, cooling the said steel to a temperature sufiiciently low to provide a martensitic structure, heating the steels within a temperature range of about 1050 F. to 1150 F. for about one to six hours, solution annealing the steel at a temperature of 1400 F. to 1550 F. for about one to eight hours, again cooling to a temperature sufiiciently low to provide a martensitic structure and thereafter aging the steels at a temperature of about 850 F. to about 950 F. for about one to ten hours.

4. The process as set forth in claim 2' wherein the minimum temperature of the second heating operation is at least 1075 F.

References Cited UNITED STATES PATENTS 3,093,519 6/1963 Decker et al. 148-31 3,131,097 4/1964 Mantel 148-143 3,132,937 5/1964 Sad-owski et al. 148-142 X 3,210,224 10/1965 Argo 148l42 DAVID L. RECK, Primary Examiner. C. N. LOVELL, Assistant Examiner. 

1. A PROCESS FOR IMPROVING THE RESISTANCE TO STRESSCORROSION CRACKING OF MARTENSITIC CAST STEELS CONTAINING ABOUT 14% TO ABOUT 17.5% NICKEL, ABOUT 8% TO ABOUT 12% COBALT, ABOUT 4% TO ABOUT 5% MOLYBDENUM, UP TO ABOUT 0.5% TITANIUM, UP TO ABOUT 0.5% ALUMINUM, UP TO ABOUT 0.05% CARBON, UP TO 0.1% ZIRCONIUM, UP TO ABOUT 3.5% CHROMIUM WITH THE SUM OF NICKEL PLUS CHROMIUM NOT EXCEEDING 17.5%, UP TO ABOUT 2% TUNGSTEN, UP TO ABOUT 0.5% COLUMBIUM, UP TO ABOUT 0.45% VANADIUM, UP TO ABOUT 0.5% TANTALUM, UP TO ABOUT 3% COPPER, UP TO ABOUT 0.3% BERYLLIUM, THE TOTAL SUM OF THE TUNGSTEN, COLUMBIUM, VANADIUM, TANTALUM, COPPER AND BERYLLIUM NOT EXCEEDING ABOUT 4%, AND THE BALANCE BEING ESSENTIALLY IRON WHICH COMPRISES SUBJECTING THE STEELS TO A HOMOGENIZATION HEAT TREATMENT WITHIN THE TEMPERATURE RANGE OF 1700*F. TO 1850*F. FOR A PERIOD SUFFICIENT TO ACHIEVE A GOOD HOMOGENEOUS STRUCTURE, COOLING THE SAID STEEL TO A TEMPERATURE SUFFICIENTLY LOW TO PROVIDE A MARTENSITIC STRUCTURE, HEATING THE STEEL WITHIN A TEMPERATURE RANGE OF ABOUT 1000*F. TO 1250*F. FOR ABOUT ONE TO EIGHT HOURS, SOLUTION ANNEALING THE STEEL AT A TEMPERATURE OF 1350*F. TO 1600*F. FOR ABOUT ONE TO TEN HOURS, AGAIN COOLING TO A TEMPERATURE SUFFICIENTLY LOW TO PROVIDE A MARTENSITIC STRUCTURE AND THEREAFTER AGING THE STEELS AT A TEMPERATURE OF ABOUT 800*F. TO ABOUT 1000*F. UP TO ABOUT 24 HOURS. 